Influence of face velocity and exhaled breath condensate on the filtration efficiency of masks and mask materials for COVID-19 transmission prevention

OBJECTIVE: Masks can be used to reduce the transmission of respiratory viruses via airborne and respiratory droplets. The objective was to evaluate the overall performance of masks and mask materials considering different use scenarios simulating real use conditions. METHODS: The filtration capability of 15 types of masks and filter materials useable as insert in masks were tested at two different face velocities (10 and 25 cm/s). Four of these masks were tested at two additional face velocities (17.5 and 32.5 cm/s). Seven of these masks were exposed to aerated simulated exhaled breath condensate (EBC) for 1-24 h and their filtration efficiency was tested immediately following exposure. RESULTS: The filtration efficiency of non-woven masks decreased by 1-17% as the face velocity of the challenge aerosol stream increased. On the other hand, an increase in the face velocity did not reduce the filtration efficiency of woven and knitted materials and of two of the commercial cotton masks. For the cloth masks, the filtration efficiency decreased by up to 20% after EBC exposure. There was no correlation between the EBC exposure and the pressure drop. CONCLUSION: The face velocity range specified for ASTM testing is too large and should be revised as the filtration efficiency can vary considerably within that range. Cloth masks should be worn with a filter and the filter be removed after a maximum of 8 h to prevent a decrease in filtration efficiency due to breath condensate accumulation.